Insulating structure



y 1945. R. R. MACHLETT ET AL 2,376,439

INSULATING STRUCTURE Filed June 18, 1945 a d M. a. 1945 l 2,376,439

UNITED .STATES PATENT OFFICE 2,376,439 INSULATING STRUCTURE Raymond B.Maehlett, New Canaan, and Joseph W. Bkehnn, Stamford, Conn., asslgnoreto Machlett laboratories Incorporated, Springdale, Com, a corporation ofConnecticut Application June 13, 1943, SerialNo. 491,888 5 Claims. (0].250-275) This invention relates to h-volta e equipsections per foot ofcolumn length. While such ment, such as high voltage electricaldischarge sectionalizing oflcrs some advantages, it is for devices, andresides more particularly in a novel from the theoretical ideal andleaves much to be insulating structure for use in separating pointsdesired. g operating at large diflerences of potential in such a Thepresent invention is, accordingly, directed equipment. The new structureis especially suited to the provision of an insulating structure forhigh for use in apparatus in which the potential along voltage use,which is more highly sectionalized the structure from one end to theother is to be and, thus, approaches the theoretical ideal moreaccurately controlled. Such constructions have closely, than hasheretofore been possible. In found application in high voltage X-raytubes 30 addition, the new structure may be sealed from and in apparatusfor nuclear research, and are the high vacuum pumps and will retain itshigh variously referred to as cascade, laminted, or vacuum indefinitely,sothat it may be employed seotionlized tubes. 1 for its intended purposewithout continuous A sectionallzed tube for generating high voltpumping.Whereas prior sealed oflf tubes have age x-rays consists of alternatemetal and inis been made with no more than three sections per sulatingmembers interposed between the clos foot, the structure of the presentinvention may trodes to which the extremes of potential are be made withthree sections per inch or even less applied, and means are provided formaintaining of column length. As a result, the total overall the metalsections at predetermined potentials dimension or a high voltage X-raytube, for exrelative to the main electrodes. Such means may an ample, inwhich the new structure is employed, take the form of connectionsbetween the metal may be safely reduced to a small fraction of thesections and appropriate points on the secondary length previouslyrequired. of the high voltage transformer supplying high The newstructure comprises a series of closely voltage to the main electrodesand resistance spaced similar thin annular discs of metallic maconnections and other expedients have also been as terial insulated fromone another by sections of suggested for the purpose. By reason or theirinsulating material, preferably a hard glass, the sectionalizedconstruction, such tubes, in general, sections being sealed vacuum-tightto the discs. may be operated at higher potentials than is pos- Themetal 01 which the discs is made is one havsible with the moreconventional construction, in 111 a coeilicient of expansion closelysimilar to which the full potential is applied to the principal so thator the material used for the insulating secelectrodes and no expedientsare provided for intions, and the surfaces of the discs in contactsuring a uniform potential gradient along the tube with the sections areoxidized so that good seals wall from one main electrode tothe other. Ithas are obtained. The discs are circumierentially also been found inhigh voltage X-ray work that distorted, as by having their outer edgesdished, accurate control of the potential along the path 5 to preventtheir being warped during the sealing of the electrons traveling betweenthe cathode of the sections thereto. By employing the sevand the targetis of considerable assistance in conare] features mentioned, the partsof the structrolling the focusing of the electron beam on the ture canbe readily assembled by the use of a target. method and apparatusinvented by us, in accord- The advantages mentioned, which are obtainedso once with which the sealing of the insulating from sectionalizing theinsulating column between sections to the discs is eilected by heattransthe main electrodes 01' high voltage apparatus, mitted'from thediscs to the sections. such as a high voltage X-ray tube, are greatlybetter understanding f the n i n. increased by increasing the number ofsections reference may be had to the accompanying drawper unit length01' the column. In practice, no 45 inginwhich limittothe improvementdcrivabletrom increased Fig. 1 is a longitudinal sectional view of ansectlonalization has been found, and it appears X-ray tube whichembodies the invention; that the practical limit imposed by mechanicalFigs. 2 and 3 are plan and sectional views, re-

considerations will be reached before the theospectively, or one or themetallic discs;

retical ideal is attained. Heretofore, the sec-- so 8 4 n 5 are P n nSectional vi w tionalizing oi "sealed-oil" tubes, that is, tubesspectively, of one of the insulating sections; and which are ca abl ofholdi g a high va uum a d ig. 6 is a front elevational view or thecathode. ire n n s p ping during oper Thetube illustrated includes thenew insulatu so far as 1 am aware, not progressed in: structure servingas the envelope oi the tube beyond the point or providing more thanthree as i consisting of a eries or thin annular discs :2

separated by tubular sections ll of insulating material, preferably ofbore-silicate glass. The discs l are made of a material having acoefllcient of expansion closely similar to that of the material ofwhich the insulating sections are made and when the sections aremade ofborosilicate glass, the discs may be made of an alloy of nickel, iron,and cobalt, known commercially as Kovar." The discs arecircumferentially stiffened to prevent their being warped when heatedand the desired stiffness may be imparted to the rings in various ways,as by pressing a circumferential channel in one face with incidentformation of a corresponding head in the other. Preferably, the discsare flat throughout the major portion of their faces, and they arestiffened by having their outer edges dished, as indicated at l2.

The glass sections II are formed with parallel fiat faces I3 and theinner diameter of the discs is substantially less than that of thesections, so that the inner edges H of the discs extend inward into thenvelope a considerable distance beyond the inner surfaces of thesections and define a substantially cylindrical surface. The dimensionsof the insulating sections are such that the discs are closely spacedand there may be as many as thre discs or more to the inch in apractical construction. The overall length of the series of discs andsections depends on the voltage to which the tube is to be subjected andone construction suitable for use in a tube to be operated at a voltagein excess of two million volts, includes 172 discs separated by glasssections and has an overall length of about 5'7".

The tube is completed by closing the series of discs or rings andsections at its ends by metallic end caps i5 and It. These end caps aresealed to insulating sections at the end of the series and each capincludes a tubular portion with a flange at its inner end.

The cap contains a metallic tube i'l secured to the inner wall thereofat its outer end and provided at its inner end with a flange l8extending outwardly close to the inner surface of cap l5. Within theinner end of tube 11 is mounted a metal block l9 having a recess inwhich is contained an incandescible filament 2|, here shown as helicalin form. Outwardly from the block 20 is a second block 22 in contactwith block IS. The lead 23 from the filament extends through an enlargedpassage 26 in block I9 and out of contact with the wall of the passageand through a plug 25 of insulating material mounted in an openingthrough block 22. The other lead 26 of the filament passes throughaligned openings in blocks 19 and 22 and is not insulated therefrom. Theleads 23, 25 pass out of the envelope through a body of glass 21 closingan opening at the end of cap it, and the leads are sealed in the glassbody. At its inner end, the cap I5 is provided with a flange 28 sealedto the end insulating section 29. It will be noted that the surface ofthe filament facing the interior of the envelope lies substantiallyparallel to a plane through the inner surface of flange 28.

The end cap l6 includes a tubular portion, at the end of which ismounted the anode 30 having a target surface of any suitable material.The cap is formed at its inner end with a flange 3i sealed to theinsulating section 32. at the adjacent end of the series.

In the formation of the device in accordance with our method abovereferred to, the end cap it, nd insulating section 32, and one of thediscs III are placed in contact and flange ll of the cap and the discare then heated by high frequency current until the heat transmittedthrough the metal to the glass causes the latter to soften and adhere tothe flange and disc. By this operation, the surfaces of the flange anddisc facing the glass are oxidized and, as the glass softens and becomesplastic, the flange and disc are caused to approach each other to assumetheir desired final space relation. In this operation, the glass sectionis likely to be slightly deformed. It will be noted that, in theconstruction shown, the outer diameter of the glass section is slightlyless than the outer diameter of the ring and flange and, as thedeformation of the glass section occurs, the glass may flow to 1111 inthe dished edge of the disc, as indicated at 33. When the cap has beensecured to the first disc by the sealing of the glass section to theparts, another section and disc are placed in position, and the glass issealed to the two discs. The operations are then repeated until a seriesof discs and sections of the desired length has been completed. Theflange 28 of end cap I5 is finally sealed to the end section 39 and thetube is thereafter subjected to the usual processing operations toevacuate it and prepare it for use.

By making use of stiffened discs and forming the wall of the envelope ofalternate discs and glass sections, the sealing together of the discsand sections by induction heating can be accomplished without distortionof the discs. During such heating, the oxidation of surfaces of thediscs permits the glass to wet the discs properly to make good seals. Asno distortion or the discs occurs during the heating, the discs can beassembled on accurate spacings and with the inner edges of the discsaligned as illustrated. In the operation of the tube, the presence ofthe discs makes it possible to cause charges to accumulate evenly on theenvelope Wall and to insure such even distribution, it is important thatthe discs be evenly spaced and concentric. Since in a tube for highvoltages, the series of discs and glass sections forming the envelopemay be of substantial length, it is also of great importance that thefaces of the discs be parallel to one another, since otherwise errorsintroduced by the nonparallel relationship might be additive and thusimpair the functioning of the discs in insuring a uniform potentialgradient along the wall.

In the operation of the tube in which the new structure is used, thediscs serve an additional purpose, namely, that of assisting in focusingthe electron beam. In any such tube, the electrons fall through theelectric field between the anode and cathode along paths which cut thelines of force of the field at right angles. If the field is uniform,the electrons will move from the cathode to the anode along straightparallel paths but variations in the field will cause distortion of thepaths. In tubes for extremely high voltages, it is of utmost importancethat the paths of the electrons b controlled and predictable to avoiddamage to the tube and, in one such tube, it is desired to have theelectrons enter a uniform field upon leaving the cathode and travel insuch a field and without divergence from straight paths until they reachthe target. With the new structure, such a field may be maintainedthroughout the length of the structure by impressing the properpotentials on the metal discs. At the anode and cathode ends of thestructure, there may be some distortion of the fleld by reason of a lackof symmetry in the construction.

nary production' methods and, in its finished form, it holds the desiredhigh vacuum indefinitely. The use of the discs sealed by the glasssections in precise geometrical relations makes it possible to insurenot only the uniform potential gradient along the envelope wall and theprevention of charge accumulations which might result in puncture, butalso the proper contribution of the discs to the focusing of the beam.

While the new structure has been specifically described in the foregoingas applied to a high voltage X-ray tube, it will be evident thatnumerous other applications in the field of electronic tubes suggestthemselves. For example, there are many instances of high voltageapparatus in which a vacuum-tight tubular duct is useful between pointsin the machine which must operate at high potential differences, and thenew structure is suitable for such use. In the case of lower voltagetubes for application in the field of communication, direction finding,and industrial electronic controls, the use of columns along which thepotential gradient can be accurately controlled is also indicated andthe new structure may be advantageously employed in such tubes.

We claim:

1. .An evacuated electric discharge device which comprises an envelopemade up of a series of closely spaced similar annular metallic discs andannular sections of glass sealed to opposite faces of the discs, eachsection joining a pair of discs together and insulating them from oneanother, the discs being made of a metal of approximately the samecoefficient of expansion as the glass and of such gauge that the discs,if flat, would be subject to warping out of their plane when heated tothe temperatures required to efl'ect the seals, the discs beingcircumferentially stiffened against such warping and having inner edgeportions extending parallel and lying inward of the glass sections,closures for the ends of the envelope, an anode within the envelopemounted on one closure, and a cathode mounted within the envelope on theother closure.

2. An insulating structure which comprises a hollow column made up of aseries of closely spaced similar annularmetallic discs and annularsections of glass sealed to opposite faces of the discs, each sectionjoining a pair of discs together and insulating them from one another,the discs being made of a metal of approximately the same coeflicient ofexpansion as the glass and of such gauge that the discs, if fiat, wouldbe subject to warping out of their plane when heated to the temperaturesrequired to effect the seals, the discs being circumferentiallystifiened against such warping and having inner edge portions extendingparallel and lying inward of the glass sections.

3. An insulating structure which comprises a hollow column made up of aseries of closely spaced similar annular metallic discs and annularsections of glass sealed to opposite faces of the discs, each sectionjoining a pair of discs together and insulating them from one another,the discs being made of a metal of approximately the same coeillcient ofexpansion as the glass and of such gauge that the discs, if flat, wouldbe subject'to warping out of their plane when heated to the temperaturesrequired to efiect the seals, the discs being circumferentiallystiffened against such warping and having inner edge portions extendingparallel and lying inward of the glass sections, the faces of the discsto which the glass is sealed being oxidized.

4. An insulating structure which comprises a hollow column made up of a,series of closely spaced similar annular metallic discs and annularsections of glass sealed to opposite faces of the discs, each sectionjoining a. pair of discs together and insulating them from one another,the discs being made of a metal of approximately the same coefficient ofexpansion as the glass and of such gauge that the discs, it flat, wouldbe subject to warping out of their plane when heated to the temperaturesrequired to effect the seals, each disc being formed with acircumferentially distorted portion lying at one side of the glass sealsand effective to prevent warpins of the disc during sealing, the discshaving inner edges lying parallel and inward of the glass sections.

5. An insulating structure which comprises an envelope made up of aseries of closely spaced similar annular metallic discs having flatportions and circumferentlally distorted portions at one side of theflat portions, and annular sections of glass sealed to the opposite flatfaces of the discs, each section joining a pair of discs together andinsulating them from one another, the discs being made of a metal ofapproximately the same coefllcient of expansion as the glass.

and of such gauge that the discs, but for the circumferentiailydistorted portions, would be sub- Ject to warping when heated to thetemperatures required to efiect the seals, the discs having parallelportions lying inward of the glass sections.

RAYMOND R. MACHLE'II. JOSEPH W, SKEHAN.

